Thermionic cathode



June 1937- H. c. STEINER ET AL 2,032,502

THERMIONI C CATHODE Filed April 26, 1929 Inventors: Harry CSteiner,Orh'n W. Livingston, by Them Attorney.

Patented June 1, 1937 UNITED STATES PATENT OFFlCE 2,082,602 THERMIONICCATHODE Harry 0. Steiner, Scotia, and Orrin W. Livingston,Schenectady,,N. Y., assignors to General Electric Company, a corporationof New York Application April 26,1929, Serial No. 358,354 11 Claims.(01. 250-275) The present invention relates to gas conduction tionalarea and for carrying the heavy current devices and more particularly tothe cathode and for providing ruggedness. In the case of structure ofsuch devices. filamentary cathodes of ordinary length which An articlein the General Electric Review, April are designed for relatively lowvoltages, these con- 1929, vol. 32, No. 4, pages 211L223, describes aditions may be satisfied by the use of metal ribform of electricaldischarge apparatus termed a bon wound as a continuous helix either as aver- Thyratron. The present invention relates to tical convolute or as asingle, flat spiral. How apparatus of this type. ever, in the event thecathode is designed for The tube described in the article referred to,.higher voltages, in order to offer the proper reis essentially athree-electrode device, into which sistance, the filament is necessarilylong on ac- 10 a small amount of an ionizable medium consistcount of itslarge cross sectional area and it is ing of an inert gas or a. vaporproducing substance evident that the amount of space taken up by the hasbeen introduced. The presence of the ionizhelical cathode of therequisite length may preable medium changes the discharge into anarcclude its advantageous employment in tubes of like discharge having afiat or slightly negative limited interelectrode space. 15 volt-amperecharacteristic. The starting of the Accordingly, one of the features ofour invenarc may be controlled electrostatically by means tioncontemplates an improved form of cathode of the grid member to which a,negative charge for controlled arc devices in which a relatively may beapplied, the discharge ceasing only by the large electron emittingsurface, constituted as a removal of the plate voltage. The removal orheavy conductor of long length, is confined in a 20 stoppage of theplate voltage conveniently is acrelatively small space and at the sametime offers complished by energizing the plate circuit with the fullmeasure of effectiveness in presenting alternating current whichperiodically drops the entire active surface to the anode. Accordthruzero. Such a tube may thus be used as an ing to another feature of ourinvention proelectrostatically controlled arc rectifier in which visionis made to separate the load current from 25 the charge on the griddetermines the point in the cathode heating current in so far as the thealternating current cycle at which the arc lead-in conductors areconcerned thus relieving discharge shall start and hence controls theavthe filament conductors of the burden of transerage value of the platecurrent. This current porting the plate current so that the size ofthese may have a magnitude almost without limit exconductors need onlybe large enough to carry 30 cept for apparatus cooling and cathodeemission the heating current. This feature is accomconsiderations; thefiow of current may be inplished by providing an additional leadconnected itially controlled by an expenditure of grid ento the centeror mid-point of the filament and ergy of less than one microwatt.forming part of the load circuit.

In view of the arc-like character of the current It has been found inthe operation of tubes of 35 stream, the various elements within thetube the kind described that when the ribbon filamust be carefullydesigned, particularly the ments are wound as a helix in the mannerprocathode. The heated member is normally subposed by the prior art theforce exerted by the jected to considerable positive ion bombardmentmagnetic field of the filament, which is of an acin the larger sizetubes and there is always prescumulative character, deleteriouslyaffects the 4 ent the danger of developing localized hot spots movementof the positive ions and electrons,

, with consequent burning out. Hence, it is imtending to defiect theseparticles from their norperative to provide a filament of relativelylarge mal path between electrodes and generally to incross sectionalarea and of rugged construction. terfere with the starting of the deviceand oper- Morever, inasmuch as there is a large plate curationthereafter. As a further feature of our in- 45 rent involved whichnecessarily passes through vention, we propose to construct the cathodein some portion of the filament after traversing the such a manner thatthere are no magnetic forces are path, there is the problem of providinga filpresent or rather such forces tend to neutralize ament lead-inconductor capable of being heroneanother and hence the current is freeto move metically sealed within the stem and of suffibetween theelectrodes without impediment. 50 cient size to accommodate the combinedheating Other objects and features will be apparent as and platecurrents. The cathode must also be the specification is perused inconnection with able to supply considerable quantities of electrons, theaccompanying drawing which represents a hence calling for a largeelectron emitting surview partly in cross section of a preferredembodface in addition to the required large cross seciment of theinvention. 55

Referring tov the drawing, numeralv i designates. an evacuated envelopewhich terminates in a press 2. Within thev envelope there are coaxiallyarranged a cathode structure, an electrostatic control member or grid 3and an anode 4. In accordance withour invention,.the-cathode consists ofa conductor,]preferably of strip material. formed as a plurality ofportions,"layers or decks 5, 6 of similar contact pin I4 and at theother end-jis-ijoined,

' "ductors 21, one of as by welding, to a cap-like shield -l5iwhich iitsover the cathode. It is appare'nt-thatinasmuch the spirals are :j'oinedtogether as the centers of and the outer terminals'are'eflectively-across the source of electromotive force the .currenttraverses one of the spirals in a direction opposite to the fiow .0!

whereby the magnetic fields of the helicessneu traiize one another. Thecathode-is preferably made both sides with electronically. activesubstances such as barium carbonate (139 603)".ortotherf-suibv ablesalts of theialkaline earth metalsinorder to increase electron emission.The coatmay be formed by applying to theurface a paste of the carbonatemixed'with suitable binder and heating the fiiamenttoa proper"temperature.

The cap member it which also may comprise nickel'is closed'at thetop,pen at-the bottom andis of a length sufiicient to extend at least tronsexcept at fairly to the lower edge'of the spiral 6. In addition to beingsupported at'the center by the lead in conductor IS the cap 'or shieldi5 is joined at the outer surface to a rod lt which is fused in the stem2 and hence is rigidly held at two places.- The function of the shieldwill be explained hereinafter. 'The.grid member 3 is fabricatedpreferably as a mesh or other perforated body of a metal' such as'anickel-chromium auoy j (nichrome) which ordinarily does not emit'elecehigh temperatures. The

grid may take the general'shape of a cylindrical cap which fits over theshield member and ex-' tends a considerable distance below the cathode,being supportedat suitable distance from the cathode structure by meansof metrically disposed rods H, which are secured to a band l8 of metaltightly embracing the stem 2. A lead-in conductor is may be attached tothe band and taken out to a contact pin 20.

The anode 4 comprises a plate member preferably of nickel which iscarbonized toJoffer a black body heat-radiating surface. In order tomake contact with the anode, a terminal 2! is aflixed to the upperextremityof the envelope l and a lead connected in any suitable mannerbetween the terminal and the anode, The envelope is of the tipless typeso-called in order to accommodate the made for evacuating the envelopethrough a tube 22 in the well 'knownmanner.

During operation, the heat produced by the filament is intense and it isdesirable to protect configuration :such. as spirals in opposedrelatlon-andsinco ductor l1. The

H may be applied cl'udes aload 28 shown but which may constitute a relayor other circuit current in the other spiral ofnickel andmay be coatedonr'oneaor trons are emitted through an equal number from two sets ofdia--,

leakage atlthe terminal, provision being to azplate 25 and supported inthe proper position by means of a wire 26 secured to the coniunction ofthe getter is well known. An inert gas, such as argon, at a pressurebetween 50-250 microns may be introduced into the envelope during andafter evacuation but preferably a drop of mercury (not shown) isemployed as the source of positive ion-producing vapor. The operatingtemperature of the device may be suchthat the mercury vapor has apressure less than 1 millimeter.

The input. or control circuit comprises con- .which is connected to thepin i4 and hence to the mid-point of the filament while the other isconnected to the grid contact 20. The outputlor controlled circuit ininthe form of resistance controlling device and a source of alternatingcurrent 29 of suitable voltage to which connection can be made by meansof a double pole switch 30.

It is found that during-operation, when the cathode and the anode areenergized by the respective sources of electromotive force, electhe openor bottom end oi the cap member l5 and an electrical discharge ofarc-like character is formed between the anode and cathode members. Thestarting of the discharge may be controlled by a negative charge on thegrid of proper potential. The cap i5 serves not only to conserve theheat generated at the cathode thereby increasing the efficiency of thismember but also to prevent the electronically active material on thecathode, i. e. barium and its oxides, from being deposited on the gridmember 2 to cause deleterious grid emission therefrom as the'result ofintense bombardment produced-by the arc. It is evident that theelectrons emitted from the grid are equivalent as far as starting thetube is concerned to the cathode and if sufilcient in number tocausestarting oi' the arc, no negativev voltage appliedto the grid canprevent it. A few microamperes of grid emission is generally suflicientto spoil control of the-tube.

An inspection of the circuit diagram will show that the load circuitthrough which a current, ordinarily of many amperes, may fiow, iselectricallyseparate from the filamentary circuit and hence no conductorwithinthe tube is required to convey the combined filament and platecurrents, By reducing the current to aminimum in the various lead-inconductors ,the tendency for stem is reduced. Furthermore, it is.lessdiiheult' 'initially to seal hermetically small diameter. in glassthan large. wire such as would be required to conduct the combinedcurrent; .The filament being formed as a doubledeck spiral in accordancewith our invention and one portion being directly placed over the other,conserves the space member is w the fullest advantage. This ad- It isvery diflicult to determine accurately the mid-tap of a filamenttransformer for What we claim as new and desire to secure by LettersPatent of the United States, is:

1. An electrical discharge device comprising an spective spirals inopposite directions and the magnetic fields produced in the two spiralswill neutralize one another.

3. An electrical discharge device comprising 5. An electrical dischargedevice comprising an evacuated envelope containing a cathode, gridmounted adjacent to and coaxial with one another and cooperating withthe grid and anode members.

6. An electrical system comprising an evacuated envelope containing acathode, a grid and of the input and output connections.

8. An electrical system comprising an evacuated envelope containing aplurality of electrodes including a cathode,

put connections.

9. An electrical discharge device comprising an envelope containingvapor-producing material, a thermionic cathode of flat spiralconfiguration,

10. An electrical discharge device comprising an envelope terminating ina stem and containmaterial, a plurality of sup- HARRY C. S'I'EINER.ORR-IN W. IIVINGSTON.

cathode.

